Flow rate monitor for fluid cooled microwave ablation probe

ABSTRACT

A microwave ablation system includes an antenna assembly configured to deliver microwave energy from a power source to tissue and a coolant source operably coupled to the power source and configured to selectively provide fluid to the antenna assembly via a fluid path. The system also includes a controller operably coupled to the power source and a sensor operably coupled to the fluid path and the controller. The sensor is configured to detect fluid flow through the fluid path and the controller is configured to control the energy source based on the detected fluid flow.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S. patentapplication Ser. No. 14/066,099, filed on Oct. 29, 2013, now U.S. Pat.No. 9,237,927, which is a divisional application of U.S. patentapplication Ser. No. 12/569,171, filed on Sep. 29, 2009, now U.S. Pat.No. 8,568,398, the entire contents of all of which are incorporated byreference herein.

BACKGROUND

1. Technical Field

The present disclosure relates generally to microwave ablationprocedures that utilize microwave surgical devices having a microwaveantenna that may be inserted directly into tissue for diagnosis andtreatment of diseases. More particularly, the present disclosure isdirected to a system and method for monitoring correct system operationof a microwave ablation system.

2. Background of Related Art

In the treatment of diseases such as cancer, certain types of cancercells have been found to denature at elevated temperatures (which areslightly lower than temperatures normally injurious to healthy cells.)These types of treatments, known generally as hyperthermia therapy,typically utilize electromagnetic radiation to heat diseased cells totemperatures above 41° C., while maintaining adjacent healthy cells atlower temperatures where irreversible cell destruction will not occur.Other procedures utilizing electromagnetic radiation to heat tissue alsoinclude ablation and coagulation of the tissue. Such microwave ablationprocedures, e.g., such as those performed for menorrhagia, are typicallydone to ablate and coagulate the targeted tissue to denature or kill thetissue. Many procedures and types of devices utilizing electromagneticradiation therapy are known in the art. Such microwave therapy istypically used in the treatment of tissue and organs such as theprostate, heart, liver, lung, kidney, and breast.

One non-invasive procedure generally involves the treatment of tissue(e.g., a tumor) underlying the skin via the use of microwave energy. Themicrowave energy is able to non-invasively penetrate the skin to reachthe underlying tissue. However, this non-invasive procedure may resultin the unwanted heating of healthy tissue. Thus, the non-invasive use ofmicrowave energy requires a great deal of control.

Presently, there are several types of microwave probes in use, e.g.,monopole, dipole, and helical. One type is a monopole antenna probe,which consists of a single, elongated microwave conductor exposed at theend of the probe. The probe is typically surrounded by a dielectricsleeve. The second type of microwave probe commonly used is a dipoleantenna, which consists of a coaxial construction having an innerconductor and an outer conductor with a dielectric junction separating aportion of the inner conductor. The inner conductor may be coupled to aportion corresponding to a first dipole radiating portion, and a portionof the outer conductor may be coupled to a second dipole radiatingportion. The dipole radiating portions may be configured such that oneradiating portion is located proximally of the dielectric junction, andthe other portion is located distally of the dielectric junction. In themonopole and dipole antenna probe, microwave energy generally radiatesperpendicularly from the axis of the conductor.

The typical microwave antenna has a long, thin inner conductor thatextends along the axis of the probe and is surrounded by a dielectricmaterial and is further surrounded by an outer conductor around thedielectric material such that the outer conductor also extends along theaxis of the probe. In another variation of the probe that provides foreffective outward radiation of energy or heating, a portion or portionsof the outer conductor can be selectively removed. This type ofconstruction is typically referred to as a “leaky waveguide” or “leakycoaxial” antenna. Another variation on the microwave probe involveshaving the tip formed in a uniform spiral pattern, such as a helix, toprovide the necessary configuration for effective radiation. Thisvariation can be used to direct energy in a particular direction, e.g.,perpendicular to the axis, in a forward direction (i.e., towards thedistal end of the antenna), or combinations thereof.

Invasive procedures and devices have been developed in which a microwaveantenna probe may be either inserted directly into a point of treatmentvia a normal body orifice or percutaneously inserted. Such invasiveprocedures and devices potentially provide better temperature control ofthe tissue being treated. Because of the small difference between thetemperature required for denaturing malignant cells and the temperatureinjurious to healthy cells, a known heating pattern and predictabletemperature control is important so that heating is confined to thetissue to be treated. For instance, hyperthermia treatment at thethreshold temperature of about 41.5° C. generally has little effect onmost malignant growth of cells. However, at slightly elevatedtemperatures above the approximate range of 43° C. to 45° C., thermaldamage to most types of normal cells is routinely observed. Accordingly,great care must be taken not to exceed these temperatures in healthytissue.

In the case of tissue ablation, a high radio frequency electricalcurrent in the range of about 500 mHz to about 10 gHz is applied to atargeted tissue site to create an ablation volume, which may have aparticular size and shape. Ablation volume is correlated to antennadesign, antenna performance, antenna impedance, and tissue impedance.The particular type of tissue ablation procedure may dictate aparticular ablation volume in order to achieve a desired surgicaloutcome. By way of example, and without limitation, a spinal ablationprocedure may call for a longer, narrower ablation volume, whereas in aprostate ablation procedure, a more spherical ablation volume may berequired.

Microwave ablation devices utilize sensors to determine if the system isworking properly. However, without delivery of microwave energy, thesensors may indicate that the probe assembly status is normal. Further,defects in antenna assemblies may not be apparent except at high powers.As such, when microwave ablation system is tested using a low powerroutine a post manufacture defect may not be apparent. This isespecially important for high power microwave ablation devices, wherefailures may result in extremely high temperatures.

Fluid cooled or dielectrically buffered microwave ablation devices mayalso be used in ablation procedures to cool the microwave ablationprobe. Cooling the ablation probe may enhance the overall ablationpattern of antenna, prevent damage to the antenna and prevent harm tothe clinician or patient. However, during operation of the microwaveablation device, if the flow of coolant or buffering fluid isinterrupted, the microwave ablation device may exhibit rapid failuresdue to the heat generated from the increased reflected power.

SUMMARY

According to an embodiment of the present disclosure, a microwaveablation system includes an antenna assembly configured to delivermicrowave energy from a power source to tissue and a coolant sourceoperably coupled to the power source and configured to selectivelyprovide fluid to the antenna assembly via a fluid path. The system alsoincludes a controller operably coupled to the power source and a sensoroperably coupled to the fluid path and the controller. The sensor isconfigured to detect fluid flow through the fluid path and thecontroller is configured to control the energy source based on thedetected fluid flow.

According to another embodiment of the present disclosure, a microwaveablation system includes an antenna assembly configured to delivermicrowave energy from a power source to tissue and a coolant sourceoperably coupled to the power source and configured to selectivelyprovide fluid to the antenna assembly via a fluid path. The system alsoincludes a controller operably coupled to the power source and acapacitive device operably coupled to the fluid path and the controller.The capacitive device is configured to detect fluid flow through thefluid path based on a capacitance of the capacitive device. Thecontroller is configured to control the energy source based on thedetected fluid flow.

According to another embodiment of the present disclosure, a method ofdetecting fluid flow through a microwave ablation system includes thesteps of delivering microwave energy from a power source to tissue viaan antenna assembly and supplying fluid from a coolant source to theantenna assembly via a fluid path. The method also includes the steps ofdetecting a capacitance of a capacitive device operably coupled to thefluid path and comparing the detected capacitance to a predeterminedrange. The method also includes the step of modifying output of energyfrom the energy source based on the comparison between the detectedcapacitance and the predetermined range.

According to another embodiment of the present disclosure, a microwaveablation system includes an antenna assembly configured to delivermicrowave energy from a power source to tissue and a coolant sourceoperably coupled to the power source and configured to selectivelyprovide fluid to the antenna assembly via a fluid path. The system alsoincludes a controller operably coupled to the power source and a sensordisposed within the fluid path and operably coupled to the controller.The sensor is configured to detect fluid flow through the fluid path.The controller is configured to control the generator based on thedetected fluid flow.

According to another embodiment of the present disclosure, a microwaveablation system includes an antenna assembly configured to delivermicrowave energy from a power source to tissue and a coolant sourceoperably coupled to the power source and configured to selectivelyprovide fluid to the antenna assembly via a fluid path. The system alsoincludes a controller operably coupled to the power source and anelectro-mechanical switch disposed within the fluid path and operablycoupled to the controller. The electro-mechanical switch is configuredto detect fluid flow through the fluid path based on movement of theswitch between an open position and a closed position. The controller isconfigured to control the generator based on the detected fluid flow.

According to another embodiment of the present disclosure, a method ofdetecting fluid flow through a microwave ablation system includes thesteps of delivering microwave energy from a power source to tissue viaan antenna assembly and supplying fluid from a coolant source to theantenna assembly via a fluid path. The method also includes detecting aposition of an electro-mechanical switch disposed within the fluid pathand modifying output of energy from the energy source based on thedetected position.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of the presentdisclosure will become more apparent in light of the following detaileddescription when taken in conjunction with the accompanying drawings inwhich:

FIG. 1 shows a diagram of a microwave antenna assembly in accordancewith an embodiment of the present disclosure;

FIG. 2 shows a perspective view of a distal end of the microwave antennaassembly of FIG. 1;

FIG. 3 shows a system block diagram of a microwave antenna assemblyaccording to another embodiment of the present disclosure;

FIG. 4 shows the area of detail of FIG. 3 according to an embodiment ofthe present disclosure;

FIGS. 5A and 5B show the area of detail of FIG. 3 according to anotherembodiment of the present disclosure;

FIGS. 6A and 6B show the area of detail of FIG. 3 according to yetanother embodiment of the present disclosure; and

FIG. 7 shows a system block diagram of a signal processing circuitaccording to an embodiment of the present disclosure.

DETAILED DESCRIPTION

Embodiments of the presently disclosed apparatus are described in detailbelow with reference to the drawings wherein like reference numeralsidentify similar or identical elements in each of the several views. Inthe discussion that follows, the term “proximal” will refer to theportion of a structure that is closer to a user, while the term “distal”will refer to the portion of the structure that is farther from theuser.

Generally, the present disclosure is directed to a microwave antennaassembly having an energy source or generator adapted to deliver energyto tissue via the antenna assembly and a coolant source for circulatinga dielectric coolant fluid through the microwave antenna assembly. Moreparticularly, the present disclosure is directed to monitoring fluidflow through the microwave antenna assembly and controlling the energysource output based on the monitored fluid flow to prevent damage to theantenna and/or harm to the clinician or patient caused by overheating ofthe antenna assembly.

FIG. 1 shows a microwave ablation system 10 that includes a microwaveantenna assembly 12 coupled to a microwave generator 14 via a flexiblecoaxial cable 16. The generator 14 is configured to provide microwaveenergy at an operational frequency from about 500 MHz to about 5000 MHz,although other suitable frequencies are also contemplated.

In the illustrated embodiment, the antenna assembly 12 includes aradiating portion 18 connected by feedline 20 (or shaft) to the cable16. More specifically, the antenna assembly 12 is coupled to the cable16 through a connection hub 22 having an outlet fluid port 30 and aninlet fluid port 32 that are connected in fluid communication with asheath 38. The sheath 38 encloses radiating portion 18 and feedline 20to form a chamber 89 (FIG. 2) allowing a coolant fluid 37 to circulatefrom ports 30 and 32 around the antenna assembly 12. The ports 30 and 32are also coupled to a supply pump 34 that is, in turn, coupled to asupply tank 36 via supply line 86. The supply pump 34 may be aperistaltic pump or any other suitable type. The supply tank 36 storesthe coolant fluid 37 and, in one embodiment, may maintain the fluid at apredetermined temperature. More specifically, the supply tank 36 mayinclude a coolant unit that cools the returning liquid from the antennaassembly 12. In another embodiment, the coolant fluid 37 may be a gasand/or a mixture of fluid and gas.

FIG. 2 illustrates the radiating portion 18 of the antenna assembly 12having a dipole antenna 40. The dipole antenna 40 is coupled to thefeedline 20 that electrically connects antenna assembly 12 to thegenerator 14. The dipole antenna 40 includes a proximal portion 42 and adistal portion 44 interconnected at a feed point 46. The distal portion44 and the proximal portion 42 may be either balanced (e.g., of equallengths) or unbalanced (e.g., of unequal lengths). A dipole feed gap “G”is disposed between the proximal and distal portions 42 and 44 at thefeed point 46. The gap “G” may be from about 1 mm to about 3 mm. In oneembodiment, the gap “G” may be thereafter filled with a dielectricmaterial at the feed point 46. The dielectric material may bepolytetrafluoroethylene (PTFE), such as Teflon® sold by DuPont ofWillmington, Del. In another embodiment, the gap “G” may be coated witha dielectric seal coating.

With reference to FIG. 2, the antenna assembly 12 also includes a choke60 disposed around the feedline 20. The choke 60 may be aquarter-wavelength shorted choke that is shorted to the feedline 20 atthe proximal end (not illustrated) of the choke 60 by soldering or othersuitable methods.

Assembly 12 also includes a tip 48 having a tapered end 24 thatterminates, in one embodiment, at a pointed end 26 to allow forinsertion into tissue with minimal resistance at a distal end of theradiating portion 18. In those cases where the radiating portion 18 isinserted into a pre-existing opening, tip 48 may be rounded or flat. Thetip 48 may be formed from a variety of heat-resistant materials suitablefor penetrating tissue, such as metals (e.g., stainless steel) andvarious thermoplastic materials, such as poletherimide, and polyamidethermoplastic resins.

With reference to FIG. 3, a microwave ablation system, shown generallyas 200, according to an embodiment of the present disclosure isdepicted. The system 200 includes an ablation device 202 having anantenna 203 and a handle 205 used to ablate tissue. A generator 206,which is substantially similar to power generating source 28, suppliesthe ablation device 202 with energy via coaxial cable 204. Ablationdevice is supplied with coolant or fluid from coolant supply 210 throughconduit 208. The coolant flows through the ablation device 202 asdescribed above and exits the ablation device via conduit 208 intochamber 214. Conduit 208 may be a multi-lumen conduit having an inflowlumen for supplying the ablation device 202 with coolant and an outflowlumen for coolant to exit the ablation device 202 into the chamber 214.Additionally, conduit 208 may be provided as two separate conduits, aninflow conduit and an outflow conduit.

As shown in FIG. 3, a sensor 212 is provided to monitor the flow rate offluid through conduit 208. As described above, when coolant circulationis interrupted, the ablation device tends to exhibit rapid failures.Further, when coolant circulation is too great and/or the flow rate offluid through conduit 208 exceeds a predetermined threshold maximum(e.g., 100 ml/min, 130 ml/min, etc.), the ablation device may beexhibiting symptoms of a fluid leak. By monitoring the fluid flowthrough microwave ablation system 200, damage to the ablation device aswell as harm to the clinician or patient may be prevented. Controller216 is coupled to generator 206 and is configured to control generator206 based on an input or signal from sensor 212. Controller 216 may be amicroprocessor or any logic circuit able to receive an input or signalfrom sensor 212 and provide an output to control generator 206.Controller 216 may be operably coupled to a storage device or memory(not shown) configured to store programmable instructions, historicaldata, lookup tables, operating parameters, etc. Sensor 212 may be asingle sensor or an array of sensors configured to detect operationalparameters of the ablation device 202 in order to determine if themicrowave ablation system 200 is functioning properly. In use, sensor212 provides an electrical signal to the controller 216 that representsa real-time measurement or indication such as fluid flow, pressure,capacitance, etc., as described in further detail below. Controller 216compares the electrical signal to a predetermined range. If theelectrical signal is within a predetermined range, the controller 216controls the generator 206 to continue with the ablation procedure. Ifthe electrical signal is outside the predetermined range, the controller216 controls the generator 206 to cease the ablation procedure and/ormodify generator 206 output. For example, the predetermined range may bea range of fluid flow rates through conduit 208. In one embodiment, thepredetermined range of flow rate requirements is between about 70 ml/minand about 130 ml/min. In this scenario, an operating range of flow ratesmay be between about 100 ml/min and about 120 ml/min. In anotherembodiment, the predetermined range of flow rate requirements is betweenabout 40 ml/min and about 100 ml/min. In this scenario, an operatingrange of flow rates may be between about 60 ml/min and about 80 ml/min.

Sensor 212 may be incorporated into ablation device 202 or controller216 may be coupled to ablation device 202 and/or controller 216. Sensor212 may be placed anywhere along the fluid path. For instance, sensor212 may be placed in antenna 203, handle 205 or along the inflow lumenor outflow lumen of conduit 208. The sampling rate of sensor 212 issufficient to detect intermittent problems with the flow of fluidthrough the ablation system 200. The sensor 212 is configured to detectfluid flow during startup before microwave energy is delivered to theablation device 202 or during an ablation procedure.

With reference to FIG. 4, a sensor 312 according to another embodimentof the present disclosure is shown operably coupled to conduit 208.Sensor 312 is configured to operate in conjunction with microwaveablation system 200 as substantially described above with reference tosensor 212. Sensor 312 is a parallel plate capacitor 314 includingcapacitive plates 322 and 324 disposed about conduit 208. Morespecifically, conduit 208 and its contents (e.g., fluid, air, airbubbles, etc.) are disposed between capacitive plates 322 and 324 tooperate as a dielectric of capacitor 314. Capacitive plates 322 and 324may be disposed about the inflow lumen of conduit 208, the outflow lumenof conduit 208, or both the inflow and outflow lumens of conduit 208.Capacitance of capacitor 314 is calculated using the following formula(1):C=ε*A/d  (1)

Applying equation (1) to the embodiment illustrated in FIG. 4, C is thecapacitance of capacitor 314, A is the area of capacitive plates 322 and324, d is the distance between capacitive plates 322 and 324, and ∈ isthe permittivity of the dielectric between capacitive plates 322 and 324(e.g., conduit 208 and its contents). Given a constant area A ofcapacitive plates 322, 324 and a constant distance d between capacitiveplates 322, 324, any change in permittivity ε of the dielectric betweencapacitive plates 322, 324, namely conduit 208 and its contents, causesa change in conductivity of capacitor 314. For example, if fluid flowthrough conduit 208 is interrupted resulting in the presence of air, airbubbles, and/or a lack of fluid flow through a portion of conduit 208disposed between capacitive plates 322, 324, the ability of conduit 208and its contents to transmit an electric field decreases, i.e., thepermittivity ∈ of conduit 208 and its contents decreases, resulting in adecrease in conductivity C of capacitor 314. That is, the presence ofair within conduit 208, rather than fluid, effectively decreases thepermittivity ∈ of the dielectric between capacitive plates 322, 324since the dielectric constant (or permittivity) of the fluid (e.g.,water) is significantly higher than air.

Sensor 312 provides an electrical signal to the controller 216indicating the capacitance C of capacitor 314 and/or the change incapacitance C of capacitor 314. Controller 216 compares the electricalsignal to a predetermined range. If the electrical signal is within thepredetermined range, the controller 216 controls the generator 206 tocontinue with the ablation procedure. If the electrical signal isoutside the predetermined range, the controller 216 controls thegenerator to terminate the ablation procedure and/or modify generator206 output.

In another embodiment, sensor 312 may be disposed about the outflowlumen of conduit 208 to detect changes in capacitance C of capacitor 314due to steam caused by overheating of ablation device 202. In thisscenario, the presence of steam in conduit 208 changes the permittivityε of the dielectric between capacitive plates 322, 324, thereby changingthe capacitance C of capacitor 314.

Referring now to FIGS. 5A and 5B, a sensor 412 according to anotherembodiment of the present disclosure is shown operably coupled toconduit 208. Sensor 412 is configured to operate in conjunction withmicrowave ablation system 200 as substantially described above withreference to sensor 212. Sensor 412 includes an electro-mechanicalswitch 420 disposed in the fluid path through conduit 208. Switch 420may be disposed within the inflow lumen of conduit 208, within theoutflow lumen of conduit 208, or within both the inflow and outflowlumens of conduit 208. Switch 420 operates to open or close anelectrical circuit electrically connected to controller 216 depending onfluid flow through conduit 208. More specifically, switch 420 is hingedat one end to a pivot 414 disposed on an inner surface of conduit 208and is configured at an opposing end to movably engage a contact 416disposed on an opposing inner surface of conduit 208 upon pivotalmovement about pivot 414 to close an electrical circuit between sensor412 and controller 216. In the illustrated embodiment, switch 420 isnaturally biased in the open position by pivot 414.

As best shown in FIG. 5A, sufficient fluid flow through conduit 208(e.g., 70 ml/min≤fluid flow rate≤130 ml/min), as depicted by directionalarrows illustrated within conduit 208, biases switch 420 to the closedposition, thereby closing the electrical circuit between sensor 412 andcontroller 216. The closed electrical circuit indicates to thecontroller 216 that sufficient fluid flow exists through conduit 208. Asbest shown in FIG. 5B, insufficient fluid flow through conduit 208allows switch 420 to return to its naturally biased open position,thereby opening the electrical circuit between sensor 412 and controller216. The open electrical circuit indicates to the controller 216 thatinsufficient fluid flow (e.g., fluid flow rate <70 ml/min) existsthrough conduit 208. As such, if switch 420 is in the closed position,i.e., contact 416 is engaged by switch 420, the controller 216 controlsthe generator 206 to continue with the ablation procedure. If switch 420is in the open position, i.e., switch 420 is disengaged from contact416, the controller 216 controls the generator to terminate the ablationprocedure and/or modify generator 206 output. Sufficient fluid flowand/or insufficient fluid flow may be dictated by a predetermined rangeof flow rate requirements depending on the ablation device 202 used.

Referring now to FIGS. 6A and 6B, a sensor 512 according to anotherembodiment of the present disclosure is shown operably coupled toconduit 208. Sensor 512 is configured to operate in conjunction withmicrowave ablation system 200 as substantially described above withreference to sensor 212. Sensor 512 includes an electro-mechanicalswitch 520 disposed in the fluid path through conduit 208. Switch 520may be disposed within the inflow lumen of conduit 208, within theoutflow lumen of conduit 208, or within both the inflow and outflowlumens of conduit 208. Switch 520 operates to open or close anelectrical circuit electrically connected between sensor 512 andcontroller 216 depending on fluid flow through conduit 208. Morespecifically, switch 520 is hinged at one end to a pivot 514 disposed onan inner surface of conduit 208 and is configured at an opposing end tomovably engage a contact 516 disposed on an opposing inner surface ofconduit 208 upon pivotal movement about pivot 514 to close an electricalcircuit between sensor 512 and controller 216. In the illustratedembodiment, switch 520 is naturally biased in the closed position bypivot 514.

As best shown in FIG. 6A, sufficient fluid flow through conduit 208(e.g., 40 ml/min≤fluid flow rate≤100 ml/min), as indicated bydirectional arrows through conduit 208, biases switch 520 to the openposition, thereby opening the electrical circuit between sensor 512 andcontroller 216. The open electrical circuit indicates to the controller216 that sufficient fluid flow exists through conduit 208. As best shownin FIG. 6B, insufficient fluid flow through conduit 208 allows switch520 to return to its naturally biased closed position, thereby closingthe electrical circuit between sensor 512 and controller 216. The closedelectrical circuit indicates to the controller 216 that insufficientfluid flow exists (e.g., fluid flow rate <40 ml/min) through conduit208. As such, if switch 520 is in the open position, i.e., switch 520 isdisengaged from contact 516, the controller 216 controls the generator206 to continue with the ablation procedure. If switch 520 is in theclosed position, i.e., contact 516 is engaged by switch 520, thecontroller 216 controls the generator to terminate the ablationprocedure and/or modify generator 206 output.

Referring to the illustrated embodiments of FIGS. 5A, 5B, 6A, and 6B,the circuitry connecting sensor 412 and/or 512 to the controller 216 forpurposes of switch state detection may include a resistor (not shown) inseries with switch 420, 520. In this scenario, the total seriesresistance of the switch 420, 520 and in-line resistor may be monitoredby the controller 216 to control generator 206 output.

With reference to FIG. 7, a signal processing circuit 300 may beincorporated within microwave ablation system 200 and electricallyconnected between sensor 212, 312, 412, 512 and controller 216. Circuit300 processes electrical signals sent from sensor 212, 312, 412, 512 tocontroller 216 for the purpose of controlling generator 206 output, asdescribed above with respect to FIGS. 4, 5A, 5B, 6A, and 6D.

More specifically, circuit 300 includes an amplifier 220 incorporatingthe sensor 212, 312, 412, 512 in a resonant feedback loop. Amplifier 220may be, for example, a Hartley Oscillator including a tuning capacitorin parallel with two inductors in series. In embodiments incorporatingsensor 312, capacitor 314 may operate as the tuning capacitor. Inembodiments incorporating sensors 212, 412, or 512, a tuning capacitorelectrically connected in series with sensor 212, 412, 512 may be addedto amplifier 220.

The output of amplifier 220 is fed into a band pass filter 222 centeredon the resonant frequency consistent with fluid flow through conduit208. That is, in the embodiment of FIG. 4, filter 222 is centered on theresonant frequency consistent with fluid flow within conduit 208 betweencapacitive plates 322, 324. In the embodiments of FIGS. 5A, 5B, 6A, and6B, filter 222 is centered on the resonant frequency consistent withfluid flow through switch 420 (FIGS. 5A and 5B) or switch 520 (FIGS. 6Aand 6B).

A sample-and-hold peak detector circuit 224 is operably coupled to thefilter 222 output such that filter 222 output is sampled and held steadyby circuit 224. Circuit 224 samples filter 222 output at a ratesufficient to detect intermittent interruptions of fluid flow throughconduit 208 and/or the presence of small air bubbles in conduit 208. Inthe embodiment of FIG. 4, absence of a peak in the filter 222 output asdetected by circuit 224 indicates air (or air bubbles) passing throughconduit 208 at capacitive plates 322, 324, as described above withrespect to FIG. 4. In the embodiments of FIGS. 5A, 5B, 6A, and 6D, apeak in the filter 222 output as detected by circuit 224 indicates theposition of switch 420 (FIGS. 5A and 5B) or switch 520 (FIGS. 6A and 6B)and/or fluid flow through conduit 208 as being sufficient orinsufficient, as described above with respect to FIGS. 5A, 5B, 6A, and6D.

The described embodiments of the present disclosure are intended to beillustrative rather than restrictive, and are not intended to representevery embodiment of the present disclosure. Various modifications andvariations can be made without departing from the spirit or scope of thedisclosure as set forth in the following claims both literally and inequivalents recognized in law.

What is claimed is:
 1. A microwave ablation system comprising: a powersource configured to generate microwave energy; an antenna assemblycoupled to the power source; a coolant source coupled to the antennaassembly and configured to supply a fluid through a fluid path to theantenna assembly; a sensor coupled to the fluid path and configured togenerate an electrical signal indicative of a flow of the fluid throughthe fluid path; a signal processing circuit coupled to the sensor andconfigured to detect at least one peak of the electrical signal beingindicative of an interruption in the flow of the fluid through the fluidpath; and a controller configured to automatically adjust output of themicrowave energy from the power source based on the detected peak. 2.The microwave ablation system according to claim 1, wherein the sensoris an electro-mechanical switch and the electrical signal is generatedbased on a position of the electro-mechanical switch.
 3. The microwaveablation system according to claim 1, wherein the sensor is a capacitivedevice and the electrical signal is generated based on a capacitance ofthe capacitive device.
 4. The microwave ablation system according toclaim 3, wherein the capacitive device comprises a dielectric disposedbetween a pair of parallel capacitive plates.
 5. The microwave ablationsystem according to claim 4, wherein the capacitance of the capacitivedevice is based on a permittivity of the dielectric disposed between thecapacitive plates.
 6. The microwave ablation system according to claim1, wherein the antenna assembly is coupled to the power source by afeedline and the antenna assembly includes radiating section connectedto the feedline.
 7. The microwave ablation system according to claim 6,wherein the antenna assembly further includes: an inlet fluid port andan output fluid port coupled to the coolant source through the fluidpath; and a sheath in fluid communication with the inlet fluid port andthe output fluid port, the sheath enclosing the radiating section andthe feedline and defining a chamber for circulating the fluid.